Most recent developments in the field of compressor construction are also focused on the undersea arrangement of large compressors, which are to serve for the delivery of natural gases. The special operating conditions, especially on account of the severely restricted accessibility both for maintenance purposes and by means of supply lines, presents great challenges to experts in this field. Relevant environmental regulations forbid any material exchange between the units which are to be installed and the surrounding seawater. There is also the fact that the seawater is an aggressive medium and extreme pressure and temperature conditions are to be encountered at different depths of the sea. Another requirement lies in the fact that the units are to have an exceptionally long service life and also have to be formed in an almost maintenance-free manner. In addition, a significant contamination of the partially chemically aggressive medium which is to be delivered makes it more difficult.
A compressor unit of the aforementioned type is already known from international patent publication WO 02/099286 A1. This document deals with the object of uncompromising simplification for reducing the maintenance cost, and with simultaneously longer service life. To achieve this object, the document proposes to design the compressor rotor in one piece with the motor rotor and to support the compressor rotor and motor rotor at the ends in each case by means of just two radial bearings. In addition, a separate thrust bearing is provided. However, with increasing overall size, it is shown that the motor rotor, which is dynamically very dominant on account of its own large weight, especially on account of thermal unbalance in the case of different operating temperatures and corresponding speeds, can cause severe vibrations which reduces the service life or unacceptably shortens the maintenance intervals.
In addition, it is known from European patent publication EP 1 074 746 B1 to equip a turbocompressor with three radial bearings, wherein the motor rotor is connected to the compressor rotor by means of a coupling. The motor rotor, which is dynamically very dominant, in this case is supported by means of two of the three radial bearings. The compressor rotor is supported at the end of the resulting common rotor by only one radial bearing. Also in this case, it is shown after a defined order of magnitude that vibrations can become comparatively large.
A compressor with a directly connected electric motor which is enclosed by a common casing are known in each case from US 2002/0037772 A1, DE 37 29 486 C1, US 2004/0179961 A1 and WO 2004/094833 A1. In each of these documents, the rotors of the electric motor and of the compressor are radially and axially supported in a different way. For example, an arrangement with four bearings and a flexible coupling between the compressor shaft and the electric motor shaft is known from these documents.